I used the paid version of "Add Watermark" by AndroidVila on Android. There is a free version too. IOS has a similar app. On myiPad, I use Scribeit (also a paid app).

Whichever app you choose, always make sure it can store or remember a frequently used text. This is important and time saving out on the field and when you have to work under the glaring sun. Also important to have freestyle one finger positioning by dragging.

Also you may like to have the apps insert a graphic or logo when necessary. Both the apps I mentioned can do that but I rarely use the feature.

Remember to keep your watermark minimal, the fonts not too fancy or big and not let it conceal and ruin the aesthetics of your pic. There are transparency and size settings to reduce distraction. It is more about signing your work like an artist or painter. Remember: No amount of watermarking can deter a determined thief.

On the desktop, I use Photoshop CC and Irfanview (free). Though I must say the mobile apps are simpler, quicker and friendlier. Hope you found this useful. Please feel free to to contact me should you have technical questions.